Residents here have been watching chromium-6 levels for years, but a recent report shows arsenic levels in a number of wells are above the state’s safe level for drinking water.

A recently completed sampling of 72 private wells in Hinkley has found that nearly 40 percent of them have arsenic levels above the state and nation’s safe drinking water level, according to John Izbicki, a U.S. Geological Survey scientist.

Izbicki is leading a five-year study of Hinkley’s groundwater to determine how much of the world’s largest chromium-6 plume formed naturally and how much is the result of Pacific Gas & Electric Co. operations in the area.

The well testing is part of the $5.4 million study.

Data from samplings of the wells taken in January found that 27 exceed the safe drinking water standard of 10 parts per billion, Izbicki said.

In the 1950s, PG&E used chromium-6 to kill microbes and provide corrosion protection for its massive cooling towers at a natural gas pumping station in Hinkley. Those towers were drained into unlined ponds, where chromium-6 percolated into the groundwater during a time when the cancer-causing properties of the chemical were not fully known.

Arsenic can be found in wells throughout the state.

Statewide, 11 percent of the groundwater used by public water operators shows arsenic levels exceeding the 10 parts per billion arsenic threshold, said Miranda Fram, a USGS chemist based in Sacramento.

Arsenic is a naturally occurring element and is harmful to the skin, digestive system, liver, nervous system and respiratory system, according to the U.S. Centers for Disease Control and Prevention website.

“Public water supply systems generally blend or treat the water to ensure that the water delivered to the consumers has arsenic concentrations below the (maximum contaminate level, or MCL),” Fram wrote in an email.

One private well in Hinkley exceeded the safe drinking limit for arsenic by nearly 30 times, Izbicki said in a phone interview Wednesday.

In addition, water from six of the 72 wells sampled exceeded the drinking water limit for uranium, while six of the 72 also exceeded the drinking water limit for nitrate, he said.

Last week, Hinkley residents received letters stating the USGS found unsafe levels of arsenic, uranium or nitrate in their wells, Izbicki said.

One Hinkley resident said she’s not afraid of the arsenic.

“I’m managing it,” Penny Harper said.

“I knew it was four times the MCL, now I find out that it is five times (the safe level for arsenic in drinking water),” she said after receiving the USGS report.

In 2013, PG&E installed a reverse osmosis filter under her bathroom and kitchen sinks and Harper said she maintains that system, which is her source of drinking and cooking water.

Harper said her chromium-6 level is only 2 parts per billion, far below California’s safe level of 10 parts per billion, which is one-tenth the federal Environmental Protection Agency standard of 100 parts per billion.

Daron Banks, another Hinkley resident who received a letter from the USGS, installed a reverse osmosis system in his house out of concerns about chromium-6, which he said would take out both chromium-6 and arsenic.

Like Harper, the Banks’ residence has chromium-6 readings well below the state’s safe level.